This invention relates to a photopolymerizable composition for cladding optical fibers.
As discussed in, for example, Blyler and Aloisio, Polymer coatings for optical fibers, Chemtech, November 1987, pages 680-684, optical fibers consist of a central core, usually a highly transparent glass (silica, often containing various doping materials) surrounded by a cladding with a refractive index lower than that of the glass; this cladding serves to confine light within the central core in order to reduce radiation losses from the surface of the core, and hence reduce attenuation of radiation travelling along the core.
In most optical fibers, the cladding is formed from a second glass. Because minor flaws in such a glass cladding greatly reduce the tensile strength of the optical fiber, it is customary to provide the fiber with a secondary, protective cladding, which is usually formed from a polymeric material. For example, U.S. Pat. No. 4,558,082, issued Dec. 10, 1985, and U.S. Pat. No. 4,663,185, issued May 5, 1987, describe acrylated silicone polymers useful as, inter alia, optical fiber claddings. These silicone polymers are prepared by reacting limonene oxide-functional silicones with acrylic acid or a substituted acrylic acid in the presence of a catalyst, which can be a tetraalkylurea or a tetraalkylguanidine.
Acrylic resins have also been used as protective claddings for optical fibers. U.S. Pat. No. 4,427,823 describes an uncured, filled coating composition comprising 100 parts by weight of (a) a polyfunctional acrylic-type carboxylic acid ester monomer or its prepolymer, this monomer or prepolymer being composed of 0 to 75 percent by weight tri- or higher acrylates, and 25 to 100 percent diacrylate; (b) 0.001 to 20 parts by weight of a polymerization initiator; and (c) 5 to 250 parts by weight of an inorganic solid filler having a refractive index of 1.40 to 1.60 and an average first-order particle diameter of at least 1 m.mu. but less than 1 .mu..
U.S. Pat. No. 4,479,984, issued Oct. 30, 1984, describes multifilament bundles (which can be optical fiber bundles) impregnated with an ultraviolet curable resin to form a composite material suitable for use as a strength member. Among the resins which can be used in such bundles are various acrylate resins.
U.S. Pat. No. 4,690,503, issued Sept. 1, 1987, describes a glass optical fiber having a primary coating constructed of two layers of ultraviolet cured acrylate resin. The first, inner layer has a modulus of elasticity at 25.degree. C. less than or equal to 5 N/mm.sup.2., while the second, outer layer has a modulus of elasticity at 25.degree. C. of from 25 to 1500 N/mm.sup.2., the ratio of the thickness of the first layer to the thickness of the second layer being between 0.5 and 2.
U.S. Pat. No. 4,572,610, issued Feb. 25, 1986, describes the cladding of optical fibers by contact with a coating composition comprising (a) a radiation-curable diethylenically unsaturated polyurethane resin constituted by an essentially saturated, halogenated, dihydroxy-terminated linear liquid polybutadiene polymer reacted with an organic isocyanate and a monoethylenically unsaturated monomer carrying a single hydroxy group, to form a diethylenic diurethane having a halogen-containing, essentially saturated polybutadiene backbone; and (b) a liquid solvent for the resin (a), this solvent being preferably a monoethylenically unsaturated liquid having a low glass transition temperature. The coating composition is cured to produce the final cladding, which has a high refractive index, above 1.48.
U.S. Pat. No. 4,469,724, issued Sept. 4, 1984, describes protecting optical fibers against stress corrosion by first coating the optical fiber with a primary coating of an ultraviolet curable, cis, trans fluoropolyolacrylate in which the modulus is reduced by eliminating about 25 percent of the pendant ester groups, curing the primary coating, applying a secondary coating of a high-modulus, heat-curable fluoroepoxy or a high-modulus ultraviolet-curable cis, trans fluoropolyolacrylate over the primary coating, and curing the secondary coating. The cis, trans fluoropolyolacrylates used must contain aromatic and oxirane rings.
However, in some cases it is possible to form the primary cladding of the optical fiber (i.e., the cladding immediately adjacent the core) from a polymeric material. According to the aforementioned Blyler and Aloisio article, polymer-clad fibers usually consist of a silica core clad with either a poly(dimethylsiloxane) resin or a fluorinated acrylic polymer. For example, U.S. Pat. No. 4,568,566, issued Feb. 4, 1986, describes photocurable silicone compositions, useful as optical fiber claddings, which compositions contain chemically combined siloxy units and units of the formula R.sub.2 SiO, where a number of the R units are acrylate or alkyl-substituted acrylate ester groupings.
U.S. Pat. No. 4,554,339, issued Nov. 19, 1985, and U.S. Pat. No. 4,597,987, issued Jul. 1, 1986, describe organopolysiloxanes having a viscosity of 100 mPa. at 25.degree. C. and having both SiC-bonded acryloxyalkyl groups and Si-bonded hydrogen atoms in the same molecule. These organopolysiloxanes are prepared by adding an allyl alcohol to a diorganopolysiloxane containing a terminal Si-bonded hydrogen atom, then esterifying the hydroxyl groups of the resultant reaction product with acrylic acid and subsequently equilibriating the resultant diorganopolysiloxane with an organo(poly)siloxane containing an Si-bonded hydroxyl group in each of the terminal units. The final organopolysiloxane is stated to be useful as, inter alia, an optical fiber cladding, although it is not clear whether this refers to a primary or secondary cladding.
However, silicone primary claddings have a number of serious disadvantages. The viscosity and curing requirements of the silicones restrict the production rate of the clad fiber to about 0.5 meters/sec. Silicone claddings do not adhere well to quartz, and the softness of the cladding leads to difficulties in connecting the clad fiber to other components of the optical system; temperature changes can cause the quartz core to be forced into and out of the cladding at the connection. Furthermore, according to U.S. Pat. No. 4,511,209, exposing the silicone-clad optical fibers to low temperatures in the range of -40.degree. to -50.degree. C. results in an increase in attenuation of 10-20 dB/km.; in many cases an increase in room temperature attenuation occurs after the fiber has been exposed to such low temperatures.
It is also known that fluorine-containing materials can be incorporated into claddings containing acrylates and methacrylates. For example, U.S. Pat. No. 4,508,916, issued Apr. 2, 1985, describes curable substituted urethane acrylates and methacrylates having an aliphatic backbone with at least one pendant fluorinated organic group attached thereto, this backbone being end-capped with an acrylic or methacrylic group.
U.S. Pat. No. 4,617,350, issued Oct. 14, 1986, describes a thermoplastic resin useful for optical purposes, including optical fiber claddings, and obtained by blending a polymer of an acrylic ester with a copolymer of vinylidene fluoride and hexafluoroacetone. The refractive index of the blend is in the range of 1.37 to 1.48.
European Patent Application Publication No. 196,212, published Oct. 1, 1986, describes a curable adhesive composition suitable for splicing optical fibers, or connecting optical fibers to other optical elements. This composition comprises a fluoroacrylate having the formula: EQU (R.sub.1).sub.2 CH(CF.sub.2).sub.n CH(R.sub.2).sub.2
wherein R.sub.1 and R.sub.2 are acrylate, methacrylate or hydrogen, n is an integer of 1 through 5, and at least one R.sub.1 is acrylate or methacrylate and at least one R.sub.2 is acrylate or methacrylate. The composition may also contain a polyfunctional acrylate or methacrylate monomer having 2 to 7 acrylate or methacrylate groups, this polyfunctional monomer allowing adjustment of the refractive index of the adhesive composition to precisely match the optical fiber refractive index. The composition may also contain an acrylate or methacrylate oligomer which does not affect the refractive index and is used to adjust the viscosity of the uncured composition to the desired level.
U.S. Pat. No. 4,511,209, issued Apr. 16, 1985 to Skutnik, describes a curable composition for use in cladding optical fibers, this composition comprising (a) a highly fluorinated monofunctional acrylate with a refractive index below 1.38 and constituting more than 50 percent by weight of the composition; (b) a trifunctional or higher acrylate that serves as a cross-linking agent; (c) a mono- or polyfunctional thiol that functions as a synergist; and (d) a photoinitiator. The resultant claddings have refractive indices in the range of about 1.39 to about 1.43.
The use of a thiol in this composition poses obvious pollution and environmental problems. Thiols are notorious for their obnoxious smells and also tend to be toxic. Although Skutnik does suggest the use of certain "low odor thiols", it would be advantageous to avoid the use of thiols entirely. In addition, to produce a composition with the viscosity needed to coat an optical fiber, it is sometimes necessary to include a solvent in the composition (some of the fluoroacrylates are solids at room temperature). The need for such a solvent imposes additional costs and environmental problems, especially if a chlorinated solvent, such as the methylene chloride used in some of Skutnik's Examples, is required. In addition, as noted by Blyler and Aloisio, supra, it is desirable to avoid solvents in cladding compositions because solvent removal is a slow process and the cladding must solidify quickly once it is applied to the optical fiber, in order that the fiber may be protected before it contacts any solid surface, such as the capstan used to draw the optical fiber.
It has also been found that very hard claddings, such as those produced by the Skutnik compositions, tend to produce microbends in an optical fiber, thus increasing the attenuation of the fiber.
There is thus a need for a photocurable composition for optical fibers which will produce claddings having suitably low refractive indices, which can be prepared without need for tri- or higher acrylates or thiols, and which produces coatings having a desirable degree of softness. This invention provides such a photocurable composition.